What are those red circles on Michael Phelps' back?

Sunday night was a big one for Michael Phelps: He won his 19th Gold Medal at the Rio Olympics, helping Team USA coast to victory in the 4x100 meter race.

SEE MORE: Everything you need to know about the Summer Olympics

While viewers were astonished by Phelps' almost super human performance, others were also wondering what the hell those circular bruises on his back were.

The big, red circles are actually the result of cupping, which is an ancient therapy used to promote blood flow and healing. According to WebMd, cupping began in ancient Chinese, Egyptian and Middle Eastern cultures and was used to treat a wide range of ailments. Cupping involves placing a heated cup upside down on the skin, creating a vacuum, which in turn causes the blood vessels in the skin to expand.

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According to the Associated Press, Phelps, 31, has been practicing cupping for while, but some doctors are wary of its supposed benefits. "I would label cupping in the category of a placebo effect," one doctor told Fox News in 2012. "It's a waste of time, it's a waste of money," he concludes.

While the benefits of cupping are up for debate, what's not is Phelps' amazing skills in the water.